Tone control circuit

ABSTRACT

A tone control circuit used in a stereo amplifier, guitar amplifier, or the like, comprising an amplifier having a first terminal and a second terminal for delivering outputs in an opposite phase relation, a plurality of circuit branches connected across the first and the second terminals for controlling tone level in the corresponding frequency ranges, and variable elements included in said plurality of circuit branches and each having a movable contact terminal connected to the ground, so that the ratio between the load impedances connected to the first and the second output terminals of the amplifier can be varied by varying the position of the movable contact terminal on either one of the variable elements, whereby the gain of the amplifier can be controlled by varying the impedance ratio in a desired frequency range.

O United States Patent 51 3,683,293

Matsui 1 Aug. 8, 1972 [54] TONE CONTROL CIRCUIT 2,093,245 9/1937 Van Loon ..330/94 [72] Inventor: Teruyoshi Matsui, Hamamatsu, FOREIGN PATENTS 0R APPLICATIONS Japan 1,556,557 12 1968 F ..330 31 [73] Assignee: Nippon Gakki Seizo Kabushiki rance Kaisha, Hamamatsu-shi, Shizuoka- Primary Examiner Roy Lake Japan Assistant ExaminerJames B. Mullins [22] Filed: June 15, 1970 Attorney-Ostrolenk, Faber, Gerb & Soffen [21] Appl. N0.: 46,288 ABSTRACT [30] Foreign Application Priority Data A tone control circuitused in a stereo amplifier, guitar amplifier, or the like, comprising an amplifier having a June 16, 1969 Japan -47412/69 first terminal and a second terminal for delivering out- June 16, 1969 Japan -47413/69 puts in an opposite phase relation, a plurality of circuit June 21, 1969 Japan ..58802/69 branches connected across firstand the Second June 21, 1969 Japan --58803/69 terminals for controlling tone level in the correspond- June 21, 1969 Japan ..58804/69 ing frequency ranges and variabue elements included in said plurality of circuit branches and each having a [52] US. Cl. ..330/31, 330/29, 330/94, movable Contact terminal connected to the ground, so 333/28 T that the ratio between the load impedances connected to the first and the Second output terminals f the [58] new of Search 313 iy plifier can be varied by varying the position of the 3 /28 /l movable contact terminal on either one of the variable elements, whereby the gain of the amplifier can be [56] Reierences Clted controlled by varying the impedance ratio in a desired UNITED STATES PATENTS frequency range- 3,566,294 2/1971 Takahashi ..330/31 X 7 Claims, 15 Drawing Figures 14 1 n l l 1 l 1 1 ii I g 15 0-|r l I g 132 PATENTEDAuc 8 m2 RESPONSE(dB) FREQUENCY (Hz) PATENTEDAu: 8 m2 SHEET 2 0F 5 FIGJ' FIG.5

FIG.6

a3 wmzoammm 1 k FREQUENCY (Hz) ATTORNEYJ P'A'TENTEDAuc a ma SHEET 3 UP 5 FIG.8

FIG.9

m3 wmZOmmmm FREQUENCY (Hz) J INVENTOR flew 4x40 M4751 TONE CONTROL CIRCUIT BACKGROUND OF THE INVENTION This invention relates generally to a tone control circuit employed in stereo amplifiers, guitar amplifiers, and the like, and more particularly to a type of circuit wherein the tone level is controlled separately in treble, bass, and if it is desired, in still other frequency ranges.

Heretofore, tone control circuits such as the CR type or NF (negative feedback) type have been widely employed in stereo and guitar amplifiers. However, the CR type tone control circuit, which will be hereinafter described in more detail, inherently causes a loss of about dB and also the input impedance thereof is substantially low, thus necessitating the employment of a driving amplifier of low output impedance at the preceding stage of the tone control circuit. Furthermore, when the driving amplifier is made of a single transistor as in ordinary cases, the gain of the transistor amplifier must be set sufficiently high for compensating the above, mentioned loss of about 20 dB, rendering it difficult to obtain a proper operational condition in view of its distortion, output impedance, operational range, and also the S/ N ratio.

Moreover, the operational characteristics of the CR type tone control circuit are not flat even at the time where the variable resistors employed respectively in the bass and treble control circuits are placed at their neutral positions. The reason for this may reside in the interference between the bass and treble. control circuits or in the production errors which affect the variation characteristics of the variable resistors. In any case, the non-flat operational characteristics of the CR type tone control circuit exhibits a deviation of several dB as described hereinafter in more detail.

Likewise, the NF type tone control circuit has a feed back circuit between the collector and the base of a transistor, and the tone control is performed by varying the frequency characteristic of the feedback circuit. However, in this case also, the input impedance of the tone control circuit is significantly lowered because of the existence of the feedback, and a separate driving circuit of low impedance is required at the preceding stage of the tone control circuit. Doubtlessly, the provision of the driving circuit will make the tone control circuit too much expensive.

SUMMARY OF THE INVENTION Therefore, a primary object of the present invention is to provide a novel type of tone control circuit in which all of the above described drawbacks of the conventional tone control circuits are substantially eliminated.

Another object of the invention is to provide a novel type of tone control circuit wherein theinterference between the treble control circuit branch and the bass control circuit branch is completely eliminated.

Still another object of the present invention is to provide a novel type of tone control circuit wherein variable resistance elements of B (the resistance varies linearly proportional to the angle) having least errors in the rotating angle-resistance value characteristic can be used and a completely flat tone control characteristic can be obtained with all of the'variable resistance elements maintained at their mechanically neutral positions.

Still another object of the present invention is to provide a novel type of tone control circuit wherein a sufficient amount of negative feedback can be maintained between the output and the input terminals of the amplifier circuit and the distortion factor of the tone control circuit is substantially reduced.

A further object of the present invention is to provide a novel type of tone control circuit wherein the output tone level can be considerably increased.

These and other objects of the present invention can be achieved by a novel construction of the tone control circuit which comprises an amplifier circuit having first and second output terminals capable of delivering outputs of opposite phase angles, a plurality of circuit branches connected across the first and the second output terminals, and variable impedance elements included respectively in these circuit branches with the movable contact terminals on the variable impedance elements connected to ground. With the above described construction of the tone control circuit, the output tone level of the amplifier circuit can be controlled separately in a plurality of frequency ranges through the plurality of circuit branches utilizing the fact that the ratio between the load impedance connected to the first and the second output terminals of the amplifier circuit can be varied by varying the position of the movable contact terminal of the variable impedance element included in each of the plurality of circuit branches.

In another aspect of the invention, the ratio of the load impedances at the time the movable contact terminals on the variable impedance elements included in the plurality of circuit branches are placed to their mechanical neutral positions is selected to be a value which is greater than one, whereby the output tone level is totally increased in accordance with the impedance ratio n.

Preferably, the amplifier circuit having a first output terminal and a second output terminal is made in the form of a transistor amplifier having a collector side output terminal and an emitter side output terminal, and the plurality of circuit branches connected across the first and the second output terminals for controlling the tone level are made in the form of a treble control circuit branch and a bass control circuit branch.

Advantageously, the treble control circuit branch comprises a resistor, a capacitor, a variable resistor having a movable contact terminal which is directly grounded, a capacitor, and a resistor, all connected in series, and the bass control circuit branch comprises a resistor, a variable resistor having a movable contact terminal directly grounded, and a resistor, all connected in series, and two series connected capacitors, the intermediate junction point thereof being directly grounded, being further connected in parallel with the above mentioned variable resistor. However, various modifications may also be constructed on the above described basic embodiment of the present invention specifically in connection with the treble and bass control circuits, and all of such modifications are intended to be considered as constituting embodiments of the present invention.

The nature, principle, and utility of the invention will be described in more detail in conjunction with the accompanying drawings wherein like parts are designated by like reference numerals.

BRIEF DESCRIPTION OF DRAWINGS In the drawings:

FIG. 1 is a circuit diagram showing an outline of the conventional CR type tone control circuit;

FIG. 2 is a circuit diagram showing an outline of the conventional NF type tone control circuit;

FIG. 3 is a typical representation of a tone control characteristic of a conventional tone control circuit such as those indicated in FIGS. 1 and 2;

FIG. 4 is a circuit diagram showing a basic example of the tone control circuit according to the present invention;

FIG. 5 shows a practicable example of the tone control circuit constituting a basic embodiment of the invention as shown in FIG. 4;

FIG. 6 is a typical representation of a tone control characteristic of the tone control circuit according to the present invention;

FIG. 7 is a circuit diagram showing a second example of the tone control circuit according to the present invention;

FIG. 8 shows a practicable example of the second embodiment of the invention;

FIG. 9 is a typical representation of the tone control characteristic of the second embodiment of the invention;

FIG. 10 is a circuit diagram showing the third example of the tone control circuit wherein the treble control circuit is simplified;

FIG. 11 shows a practicable example of the third example;

FIG. 12 is a circuit diagram showing the fourth example of the tone control circuit wherein the treble control circuit is further simplified;

FIG. 13 shows a practicable example of the fourth example; I

FIG. 14 is a circuit diagram showing the fifth example of the tone control circuit wherein the treble and also the bass control circuits are simplified; and

FIG. 15 shows a practicable example of the fifth example of the tone control circuit.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIGS. 1 and 2, there are indicated out-.

lines of the conventional CR type and NF type tone control circuits, respectively, which have been cited in the forward part of this specification. As is apparent from these figures, the CR type tone control circuit may cause a loss of about dB and the input impedance thereof may also be substantially low. Likewise, the NF type tone control circuit shown in FIG. 2, wherein a negatively sensed feedback is provided from the collector side to the base of a transistor, may also have a low input impedance because of the existence of the feedback circuit, and in either of the cases, a separate driving circuit (not shown) of low output impedance type must be provided in the previous stage of the tone control circuit, rendering the production cost of the tone control circuit to be extremely high. Furthermore, the operational characteristics of the CR type tone control circuit and the NF type tone control circuit cannot be flat, and a deviation of several dB, as indicated by y in FIG. 3 is inevitably created along the characteristic curve even if the movable contact terminals in the treble and bass control circuits are maintained at their neutral positions.

In contrast with the conventional tone control circuits, a tone control circuit indicated in FIG. 4, which constitutes a basic embodiment of the present invention, FIG. 5 being the practical example thereof, has a unique construction generally comprising an amplifier circuit 10, a treble control circuit 20, and a bass control circuit 30.

The amplifier circuit 10 further comprises a transistor 11 having a collector resistor 12 and an emitter resistor 13. An input terminal 1, an output terminal 2, and a power source terminal 3 are provided for the amplifier 10, and the amplifier 10 is coupled through capacitors 14 and 15 of sufficiently large capacitances to the first output terminal 2 and the second output terminal 2'. The treble control circuit 20 includes a resistor 21, a capacitor 22, a variable resistor 23, a capacitor 24, and a resistor 25, all connected in series, and two ends of the series connected circuit are connected to the coupling capacitors 14 and 15. A movable contact terminal A of the variable resistor 23 is directly grounded.

The bass control circuit 30 comprises a resistor 31, a variable resistor 34, and another resistor 35, all connected in series, and two series connected capacitors 32 and 33, the intermediate junction point therebetween is directly grounded, are connected in parallel with the variable resistor 34 so that the directly grounded intermediate point of the capacitors 32 and 33 is connected to a movable contact terminal B of the variable resistor 34.

In all of the treble control circuits 20 and the bass control circuits 30 of this embodiment, the resistance values of the resistors 21 and 25, capacitance values of the capacitors 22 and 24 are made equal between each other, and the resistance values of the resistors 31 and 35, and capacitance values of the capacitors 32 and 33 are respectively made equal.

In operation, when a flat characteristic of tone control is desired, the movable terminals A and B of the variable resistors 23 and 34 are placed at their neutral positions (so that the resistance between A A is equal to the resistance between A A and the resistance between B B is equal to the resistance between B B Supposing that the resistance of the load connected to the output terminal 2 is far larger than the collector resistor 12, the impedances connected to the collector side and the emitter side of the transistor 11 are substantially equal, and outputs of equal amplitudes and opposite phase angles can be obtained from the collector and the emitter. In view of the fact that the signal level obtained at the emitter side terminal 2 is approximately equal to the input signal level at the terminal 1, it is apparent that the output level at the terminal 2 is also equivalent to that of the input signal. For this reason, when the variable terminals A and B of the variable resistors 23 and 34 are placed to their neutral positions, a flat control characteristic as shown by the curve b n e in FIG. 6 can be obtained.

In the case where a characteristic of treble-boost is desired to be obtained, the movable terminal A of the variable resistor 23 in the treble control circuit branch 20 is placed at the extremity A In this condition, the position of the variable terminal B of the variable resistor 34 in the bass control circuit branch 30 causes no influence on'the treble control circuit because of the sufficiently large capacitances of the capacitors 32 and 33 which effectively short-circuit the variable resistor 34 in the treble frequency range. Furthermore, in this case, the impedance of the treble circuit observed from the terminal 2, that is the total impedance of the resistor 21, capacitor 22, and the variable resistor 23, becomes larger than the impedance of the same observed from the terminal 2', that is the total impedance of the resistor 25 and capacitor 24, whereby the collector side load impedance of the transistor 11 in the treble frequency range is larger than the emitter side load impedance of the same transistor and a control characteristic of b n d in FIG. 6 can be thereby obtained.

When a control characteristic of treble-cut is desired, the sliding terminal A of the variable resistor 23 is shifted to the A1 side. In that case, the impedance viewing from the terminal 2 becomes smaller than that viewing from the terminal 2' in reverse to the case of the treble boost, and the control characteristic of b n fin FIG. 6 is thereby obtained.

Thus, with the sliding terminal A of the variable resistor 23 shifted between the A1 side extremity and the A2 side extremity, the control characteristic of the tone control circuit can be continuously changed, in its treble frequency range, from one indicated by b n f to the other indicated by b n d as shown in FIG. 6.

In the case where a control characteristic of bass boost is desired, the sliding terminal B of the variable resistor 34 is shifted to the B2 side extremity. In this case, the positioning of the sliding terminal A for the variable resistor 23 in the treble control circuit does not influence the bass control circuit, because of the sufficiently small capacitance values of the capacitors 22 and 24, which causes the impedance of the treble control circuit observed from the terminals 2 and 2 to be sufficiently high in the bass frequency range, and the treble control circuit is effectively opened (or blocked).

With the sliding terminal B of the variable resistor 34 shifted to the B2 side extremity as described above, the impedance of the bass control circuit observed from the terminal 2, that is the resultant impedance of the resistor 31, capacitor 32, and the variable resistor 34 becomes larger than the impedance of the same circuit observed from the terminal 2, that is the impedance of the resistor 35, and as a result, the collector impedance of the transistor 11 becomes larger than the emitter load impedance of the same in the bass frequency range, whereby the control characteristic of a n e shown in FIG. 6 is obtained.

When the bass-cut control characteristic is desired, the sliding terminal B of the variable resistor 34 is shifted to the B1 side extremity. In that case, reversing to the above description, the impedance of the bass control circuit observed from the terminal 2 becomes smaller than the impedance of the same observed from the terminal 2, and the control characteristic 0 n e shown in FIG. 6 is obtained.-

Herein, if the sliding terminal B of the variable resistor 34 is moved from the B1 side to the B2 side continuously, the bass control characteristic of the tone control circuit can be continuously varied from the one indicated by c n e to the other indicated by a n e in FIG. 6.

Summarizing the above described features of the tone control circuit according to the present invention, it is apparent that, when the sliding terminals A and B of the variable resistors 23 and34 are shifted suitably, the control characteristic of the tone control circuit can be adjusted to any desired format within the range bounded by two curves of a n d and c n f shown in FIG. 6. Furthermore, the variation width of the characteristic and its turn-over and turn-ofi frequencies can be set to suitable values by suitably determining the circuit constants such as resistors and capacitors in the treble and bass control circuits.

As is apparent from all of the descriptions with respect to the basic construction of the tone control circuit according to the present invention, the fundamental principle of the present invention is in that there is utilized a characteristic feature of an amplifier consisting of a single transistor wherein the amplifying factor is determined by a ratio between the collector side load impedance and the emitter side load impedance, and also in that there are provided control circuit branches such as treble and bass control circuits each including a variable resistor for varying the above mentioned load impedance ratio, so that a tone level can be independently controlled by varying the sliding positions of the variable resistors included in these circuit branches.

Another example of the tone control circuit which is a modification of the above described basic construction of the invention is indicated in FIG. 7, and FIG. 8 shows a practical embodiment thereof. An amplifier generally designated by a numeral 10 is composed of a transistor 11 connected with a collector resistor 12 and an emitter resistor 13, both of which are connected respectively with coupling capacitors 14 and 15 of sufficiently large capacitances. As a specific feature of this example, the resistance value of the collector resistor 12 is determined to a value n-times larger than the resistance value of the emitter resistor 13 (herein n 1).

Furthermore, the amplifier circuit 10 is provided with an input terminal 1, an output terminal 2, and a power source terminal 3 as in the case of the previous example shown in FIG. 4.

This example of the tone control circuit is further provided with a treble control circuit 20 and a bass control circuit 30 as in the basic embodiment shown in FIG. 4, and the treble control circuit 20 comprises a resistor 21, capacitor 22, variable resistor 23, capacitor 24, and a resistor 25, all connected in series. The sliding terminal A of the variable resistor 23 is grounded and both ends of the series connected treble control circuit 20 are connected respectively to the above mentioned coupling capacitors l4 and 15. Herein it should be noted that, in this particular example of the tone control circuit, the resistance value of the resistor 21 is selected to be n-times larger than that of the resistor 25, and the capacitance value of the capacitor 22 is selected to be l/n of the value of the capacitor 24. Furthermore, the characteristics of the variable resistor 23 are selected to be such that, when the sliding terminal A of the variable resistor 23 is placed to its mechanically neutral position, the resistance value of its portion AA is n-times larger than that of the portion AA2.

The bass control circuit 30 in this embodiment of the invention comprises a series resistor 31, a parallel connected circuit consisting of a series connected capacitors 32 and 33 and of a adjustable resistor 34, and a series resistor 35. The variable contacting terminal B of the variable resistor 34 and the intermediate point between the capacitors 32 and 33 are connected together and the connecting point is further connected to the ground. Both ends of the bass control circuit 30 are connected to the previously mentioned coupling capacitors 14 and 15. Herein also, the resistance value of the resistor 31 is selected to be n-times larger than that of the resistor 35, and the capacitance value of the capacitor 32 is selected to be l/n of the value of the capacitor 33. Furthermore, the characteristics of the variable resistor 34 are selected to be such that, when the sliding terminal A of the variable resistor 34 is placed to its mechanically neutral position, the resistance value of the portion BB is n-times larger than that of the portion BB In the operation of this embodiment, when it is desired to obtain a flat characteristic of the output tone level, the variable terminals A and B of the variable resistors 23 and 34 are placed at their mechanically neutral positions. In this case, if the resistance value of the load (not shown) connected to the output terminal 2 is far larger than that of the collector resistor 12, the impedance connected to the collector side of the transistor 11 will be n-times larger than the impedance connected to the emitter side of the same transistor 11. When the impedance at the collector side of the transistor 11 is n-times larger than that of the emitter side thereof, an output signal obtained at the collector side of the transistor 11 will have a value n-times larger than the output signal obtained at the emitter side of the transistor, and these output signals will be in a phase relationship opposing against each other. Furthermore, since the output signal obtained at the output terminal 2 is approximately equal to the input level at the input terminal 1, the amplitude of the output signal obtained at the output terminal 2 will be ntimes larger than that of the input signal at the input terminal 1. For this reason, when the sliding contact terminals A and B of the variable resistors 23 and 34 are at their mechanically neutral positions, a characteristic represented by b o e in FIG. 9, which is flat in all of the frequency ranges and having a gain n-times larger than the input level, can beobtained.

Whenever a characteristic of treble-boost is desired, the sliding contact terminal A of the variable resistor 23 in the treble control circuit 20 is brought to the extremity of A side, and a characteristic represented by b d in FIG. 9 is obtained.

If a characteristic of treble-cut is desired, the sliding contact terminal A of the variable resistor 23 in the treble control circuit 20 is brought to the extremity of A side, and a characteristic represented by b 0 f in the same figure is obtained.

It will be apparent that when the sliding contact terminal A of the variable resistor 23 is shifted from the above described A side to the previously described A side, the tone characteristic in the treble range can be continuously varied from that represented by b 0 f to the other represented by b 0 d.

Furthermore, the bass-boost and bass-cut characteristics can be obtained by shifting the sliding contact terminal B of the variable resistor 34 in the bass control circuit 30 to B side extremity and the B side extremity, respectively, in the similar manner as described before in the case of the basic embodiment shown in FIG. 4. And, in these cases, the tone characteristics indicated by a 0 e and c 0 e, respectively, can be obtained.

As is apparent from the above description, the tone control circuit according to the present invention utilizes the fact that the amplification factor of an amplifier circuit consisting of a single transistor is determined by the ratio between the load impedance at the collector side and the load impedance at the emitter side of the transistor, and, by varying these impedances in the treble and bass frequency ranges of the input signal employing variable resistors corresponding to these frequency ranges, voltage gains of the amplifier circuit in the treble and bass ranges are independently controlled. As a particular feature of this embodiment, the load impedance at the collector side of the transistor is selected to be n-times larger than that of the emitter side at the time a flat control characteristic is maintained, and a voltage gain equalling to n is obtained at such a flat control time.

Thus the advantageous features of the basic embodiment shown in FIG. 4 and another embodiment shown in FIG. 7 above can summarized as follows.

Firstly, the sliding contact terminals of the variable resistors included in the treble and bass control circuits are all grounded, and no output signals are taken out of these sliding contact terminals and mixed as in the cases of the conventional CR type and NF type tone control circuits, whereby any interference which might be existed between the output signals can be totally eliminated.

Secondly, by determining the emitter side impedance of the transistor to be higher than a predetermined value even in the cases where the above described treble-boost or bass-boost control is undertaken, a considerable amount of current feedback is always obtained, and whereby the input impedance viewed from the input terminal 1 can be maintained to be higher than a predetermined amount.

Thirdly, by restricting the gain of the transistor amplifier to less than 12 dB even at the boost time of the control frequency range, the amplifier can be operated in a condition having a sufiicient amount of negative feedback, whereby the distortion factor of the amplifier can be reduced to a sufficiently lower value.

In addition, in the latter embodiment of the present invention, since a certain gain is assured in contrast with the conventional CR or other type of the tone control circuit wherein about 20 dB of loss in usual, the S/N ratio and the dynamic operation range of the amplifier can be substantially improved.

A practicable example of the embodiment shown in FIG. 7 is illustrated in FIG. 8. With this construction of the tone control circuit, the characteristics shown in FIG.' 9 have been obtained.

Still another example of the basic tone control circuit according to the invention is shown in FIG. 10. This example is a modification of the basic construction of FIG. 4, wherein two resistors in the treble control circuit are omitted, and a resistor is instead added between the sliding contact terminal and the ground.

To be more particular, the treble control circuit 20 of this embodiment comprises a capacitor 22, a variable resistor the sliding contact of which is grounded through a resistor 26, and another capacitor 24, all connected in series, and both ends of the series connected treble control circuit 20 are connected to the coupling capacitors 14 and 15 as described in the previous embodiment. The capacitors 22 and 24 are selected to have an equivalent value.

With this construction, when the sliding contact terminal A of the treble control circuit is placed at the neutral position, the resultant impedance of the circuit consisting of the capacitor 22, a half of the variable resistor 23, and a resistor 26 is equal to the resultant impedance of the circuit consisting of the capacitor 24, a half of the variable resistor 23, and the resistor 26, whereby a flat characteristic of the tone level in the treble frequency range can be obtained. Furthermore, when a treble-boost or treble-cut tone control is desired, the sliding contact terminal A is shifted to A or A side respectively. The operation of the bass control circuit 30 remains same as in the previous embodi ments.

FIG. 11 shows a practicable example of the tone control circuit constituting this embodiment of the invention.

Still another embodiment of the present invention, wherein the treble control circuit is further simplified is shown in FIG. 12. In this embodiment, two resistors 21 and and two capacitors 22 and 24 in the basic embodiment shown in FIG. 4 are omitted, and a capacitor 26 and a resistor 27 are connected in series between the sliding contact terminal A and the ground. With this construction, it is apparent that an operation similar to the treble control circuit 20 of the basic construction in FIG. 4 or of an example shown in FIG. 10 may also be obtained.

FIG. 13 shows a practicable example of the tone control circuit belonging to the embodiment shown in FIG. 12, and with this construction, similar tone control characteristics as indicated in FIG. 5 have been obtained.

In the above described embodiment shown in FIG. 12, the bass control circuit 30 may further be modified as shown in FIG. 14. In this construction, the bass control circuit 30 comprises a series connected resistor 31, a variable resistor 34 having a slidable contact terminal B directly connected to the ground, and another series connected resistor 35, with a capacitor 36 being connected in parallel with the variable resistor 34. In other words, two series connected capacitors 32 and 33 in the previous embodiment are replaced by onecapacitor 36 and the grounded connection of the junction point between the two capacitors 32 and 33 is omitted. Both ends of the bass control circuit 30 are connected to the coupling capacitors 14 and 15 as in the cases of the previously described embodiments.

A practicable example of this embodiment is shown in FIG. 15, and it has been found that the circuit of FIG. 15 can exhibit tone control characteristics as shown in FIG. 6, which has been obtained by the basic embodiment of the invention shown in FIG. 4.

Although the present invention has been described with respect to preferred embodiments thereof, it will be apparent that various modification may still be considered without departing the scope and spirit of the present invention. For instance, the transistors of an ordinary type which have been employed in this invention may also be replaced by those of field effect type, and the total of the tone control circuit branches may also be increased in number beyond the above described treble and bass control circuits, adding certain band control circuits including band-pass filters at portions corresponding to the low-pass or high-pass filters in the indicated circuit branches.

Iclaim:

l. A tone control'circuit comprising an amplifier circuit having a first output terminal. and a second output terminal for delivering outputs in opposing phase relation, a plurality of circuit branches connected across the first and the second output terminals for controlling the tone level in the corresponding frequency ranges, and variable impedance elements included respectively in said plurality of circuit branches and each of the variable impedance elements having a movable contact tenninal connected to ground, so that the ratio between the load impedances connected to the first and the second output terminals of the amplifier circuit can be varied by varying the position of the movable contact terminal on either one of the variable impedance elements included in the plurality of the circuit branches, whereby the gain of the amplifier circuit is allowed to be controlled in accordance with the variation of the impedance ratio in either of the frequency ranges; said amplifier being in the form of a transistor amplifier having a first output terminal at the collector side and a second output terminal at the emitter side; said plurality of circuit branches connected across the first and the second output terminals for controlling the tone level are made into two circuit branches one being a treble control circuit branch and the other being a bass control circuit branch; said bass control circuit branch comprising a first resistor, a variable resistor having a movable contact terminal, and a second resistor, all connected in series across the first output terminal and the second output terminal of said transistor amplifier, and a conductive circuit connected in parallel across the fixed terminals at the opposite ends of said variable resistor, said conductive circuit including at least one capacitor.

2. A tone control circuit as defined in claim 1 wherein the ratio of said first and! second resistors, at the time the movable contact terminal on said variable resistor element is placed at the mechanical neutral position, is selected to be a value which is greater than 1.

3. A tone control circuit as defined in I claim 1 wherein said treble control circuit branch comprises two capacitors and a variable resistance element having a sliding contact terminal, all connected in series across the first output terminal at the collector side and the second output terminal at the emitter side, and a resistor connected between the slidable contact terminal of the variable resistance element and ground.

4. A tone control circuit as defined in claim 1 wherein said treble control circuit branch comprises a variable resistance element having a sliding contact terminal, said variable resistance element connected across the first output terminal at the collector side and the second output terminal at the emitter side and a resistor and a capacitor connected in series between the variable contact terminal of the variable resistance element and ground.

5. A tone control circuit comprising an amplifier having a first output terminal and a second output terminal for delivering outputs in opposing phase relation; a treble control circuit branch including a variable resistor having a sliding terminal and two fixed terminals and at least one capacitor and one resistor, said sliding terminal being grounded, said fixed terminals being connected respectively to said first and second output terminals, and said at least one capacitor and one resistor being inserted in series between said first output terminal and ground; and a bass control circuit branch including a second variable resistor having a sliding terminal and two fixed terminals, two resistors and at least one capacitor, said sliding terminal being grounded,

said fixed terminals being connected respectively through each of said resistors to said first and second output terminals, and said at least one capacitor being connected across the two fixed temiinals of said second variable resistor.

6. A tone control circuit as defined in claim 5 wherein said treble control circuit branch includes a second capacitor and a second resistor, said sliding terminal being grounded directly, the remaining one of said fixed terminals being connected to said second output terminal through a series path comprising said second capacitor and said second resistor.

7. A tone control circuit as defined in claim 5 wherein said bass control circuit branch includes a second capacitor connected in series with said one capacitor with said series path extending across said two fixed terminals of said second variable resistor, and the juncture of said two capacitors being grounded. 

1. A tone control circuit comprising an amplifier circuit having a first output terminal and a second output terminal for delivering outputs in opposing phase relation, a plurality of circuit branches connected across the first and the second output terminals for controlling the tone level in the corresponding frequency ranges, and variable impedance elements included respectively in said plurality of circuit branches and each of the variable impedance elements having a movable contact terminal connected to ground, so that the ratio between the load impedances connected to the first and the second output terminals of the amplifier circuit can be varied by varying the position of the movable contact terminal on either one of the variable impedance elements included in the plurality of the circuit branches, whereby the gain of the amplifier circuit is allowed to be controlled in accordance with the variation of the impedance ratio in either of the frequency ranges; said amplifier being in the form of a transistor amplifier having a first output terminal at the collector side and a second output terminal at the emitter side; said plurality of circuit branches connected across the first and the second output terminals for controlling the tone level are made into two circuit branches one being a treble control circuit branch and the other being a bass control circuit branch; said bass control circuit branch comprising a first resistor, a variable resistor having a movable contact terminal, and a second resistor, all connected in series across the first output terminal and the second output terminal of said transistor amplifier, and a conductive circuit connected in parallel across the fixed terminals at the opposite ends of said variable resistor, said conductive circuit including at least one capacitor.
 2. A tone control circuit as defined in claim 1 wherein the ratio of said first and second resistors, at the time the movable contact terminal on said variable resistor element is placed at the mechanical neutral position, is selected to be a value which is greater than
 1. 3. A tone control circuit as defined in claim 1 wherein said treble control circuit branch comprises two capacitors and a variable resistance element having a sliding contact terminal, all connected in series across the first output terminal at the collector side and the second output terminal at the emitter side, and a resistor connected between the slidable contact terminal of the variable resistance element and ground.
 4. A tone control circuit as defined in claim 1 wherein said treble control circuit branch comprises a variable resistance element having a sliding contact terminal, said variable resistance element connected across the first output terminal at the collector side and the second output terminal at the emitter side and a resistor and a capacitor connected in series between the variable contact terminal of the variable resistance element and ground.
 5. A tone control circuit comprising an amplifier having a first output terminal and a second output terminal for delivering outputs in opposing phase relation; a treble control circuit branch inclUding a variable resistor having a sliding terminal and two fixed terminals and at least one capacitor and one resistor, said sliding terminal being grounded, said fixed terminals being connected respectively to said first and second output terminals, and said at least one capacitor and one resistor being inserted in series between said first output terminal and ground; and a bass control circuit branch including a second variable resistor having a sliding terminal and two fixed terminals, two resistors and at least one capacitor, said sliding terminal being grounded, said fixed terminals being connected respectively through each of said resistors to said first and second output terminals, and said at least one capacitor being connected across the two fixed terminals of said second variable resistor.
 6. A tone control circuit as defined in claim 5 wherein said treble control circuit branch includes a second capacitor and a second resistor, said sliding terminal being grounded directly, the remaining one of said fixed terminals being connected to said second output terminal through a series path comprising said second capacitor and said second resistor.
 7. A tone control circuit as defined in claim 5 wherein said bass control circuit branch includes a second capacitor connected in series with said one capacitor with said series path extending across said two fixed terminals of said second variable resistor, and the juncture of said two capacitors being grounded. 